learning to repair hardware is always a tough thing. Apple offers multiple certifications ranging from help-desk to full fledged applecare certified on all gear including laptops. I am sure to be digi certified you have to work for them.
But on the software side of things learning to fix Macs and how they work with digi hardware is pretty easy of you work with them day in and day out. I find that the most fixing you do is when someone first buys a system and has no idea how to hook it up, generally you will see someone shell out thousands on an HD3 rig and then hook up the cards in the wrong order. You will find that you can fix most everyones problems through some basic troubleshooting, and the ones you can't fix, you should be able to isolate where the problem is.
For instance when someone says their mac doesn't work, or it won't turn on you need to know what is wrong, if the light comes on but there is no startup chime could be bad RAM, or misallocation of RAM. If the light comes on then quickly off with no startup chime then it's the logic board and probably just needs to be reset. Or maybe the computer chimes and stays on but the monitor stays off, then it could be the video card or the monitor. You get the idea.
To learn more about the apple side of things check out their website:
http://www.apple.com/training/
if you don't want to shell out money for training and official certification just browse their answerbase, same goes with DIGI, figure out the most common DAE errors and what causes them.
-Roy